This invention pertains to apparatus and electrical circuits with a plurality of transformers, useful as mixers, modulators, phase detectors, limiters, and for other purposes.
Many transformers have cores which are toroid or balun shaped. The use of a toroid or balun has been recommended because of its ability to confine substantially all of the magnetic field within the toroid or balun itself. Not only is this efficient, but the confining of the electromagnetic field prevents undue electromagnetic interference to surrounding elements near the transformer. Preventing interference between electrical elements is particularly important when the surrounding elements are themselves electromagnetic in nature. Thus, it is advantageous to prevent interference between transformers in the same circuit. An additional advantage of toroid and balun cores are their capability of performing well in a large bandwidth.
However, manufacture of toroid and balun core transformers is awkward and expensive. For example, it is tedious to wind wires around a toroid or balun since the wire must be repeatedly looped in and out of the central hole or holes. The procedure of winding a balun or toroid core becomes more difficult as the core and the central hole become smaller. Further, it is difficult to affix a toroid structure transformer to a circuit board or other mounting means because it has no flat edges. A balun structure is also difficult to mount if the windings are disposed upon the only flat surfaces. Additionally, once a toroid or balun transformer is in place, its shape causes its point of connection with the circuit board to be mechanically unstable.
Other arrangements have been proposed for preventing electromagnetic interference between elements. Hassel, U.S. Pat. No. 2,021,060, discloses that for the various transformers in each stage of an amplifier, it is desirable to avoid the electromagnetic coupling caused by leakage lines of force between the different transformers in the different stages. Hassel teaches that two well known ways of preventing such electromagnetic coupling are to shield the transformers or arrange their axes at right angles to each other. Other references which disclose shielding the transformers of different stages include Braden, U.S. Pat. No. 2,065,884, and Wheeler, U.S. Pat. No. 2,075,683.
Despite this art, there is still need for further improvement.